Say how the transmit pane works and what would make it exact

The pump is paced off the clock. XMMT.ocx has a TxBufLen property and answers
when it is asked, so the bridge could report what the engine still holds and
make the red text exact, but the engine does not start on this machine and the
property could not be watched while a message was going out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

docs/unfinished.md

@@ -39,9 +39,19 @@ hardware TNCs — are not here either, though `DigitalEngine` is the place to ad
 them.
 
 The receive pane has been driven with a stand-in bridge that prints prepared
-text, which is how the printing, the colouring and the Letters/Figs box were
-checked. Nothing has been decoded from a real signal: this machine has no sound
-card.
+text, which is how the printing, the colouring, the Letters/Figs box and the
+type-ahead transmit pane were checked. Nothing has been decoded from a real
+signal and nothing has gone on the air: this machine has no sound card, and
+MMTTY does not start on it.
+
+**What paces the type-ahead pump.** It counts character times off the clock at
+the baud rate in the digital settings, so it runs a little behind the engine
+rather than ahead of it. The engine can be asked instead: `XMMT.ocx` has a
+`TxBufLen` property, and a probe against the control shows it answers, though
+what it answers while a message is going out could not be measured here because
+the engine will not start. Reporting it from the bridge would make the red text
+exact and let the pump keep the engine's buffer full rather than leaving a gap
+between characters.
 
 **Voice keying.** `MessageSender` was written to cover a voice keyer playing a
 recording, and nothing implements it. Each operator's recordings folder is